1. Field of the Invention
This invention relates to an EL (electroluminescence) display device (a light emitting device or a light emitting diode or OLED (Organic Light Emission Diode) formed by fabricating semiconductor elements (elements using a thin semiconductor film) on a substrate, and to an electronic device which uses the EL display device for a display unit thereof. The EL devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.
2. Related Art
In recent years, technology for forming TFTs on a substrate has greatly advanced and study has been forwarded to apply the technology to the active matrix-type display devices. In particular, study has been vigorously forwarded concerning the active matrix-type EL display device using EL elements as spontaneously light-emitting elements among the active matrix type display devices. The EL display device is also called organic EL display (OELD) or organic light-emitting diode (OLED).
Unlike liquid crystal display devices, the EL display device is the one that spontaneously emits light. The EL element has a structure in which an EL layer is held between a pair of electrodes, the EL layer being, usually, of a laminated layer structure. The laminated-layer structure can be represented by a “positive hole-transporting layer/light emitting layer/electron-transporting layer” proposed by Tang et al. of Eastman Kodak Co. This structure features a very high light-emitting efficiency, and almost all of the EL display devices that have now been studied and developed are employing this structure.
Luminescence of the organic EL material stems from the emission of light (fluorescence) of when a singlet excited state returns back to the ground state or stems from the emission of light (phosphorescence) of when a triplet excited state returns back to the ground state. The EL element of this invention may utilize either one of the above-mentioned type of light emission or may utilize both of the above-mentioned types of light emission.
In addition to the above, there can be further employed a lamination of positive hole-injection layer/positive hole-transporting layer/light-emitting layer/electron transporting layer or a lamination of positive hole-injection layer/positive hole-transporting layer/light-emitting layer/electron-transporting layer/electron-injection layer on the pixel electrode. The EL layer may be doped with a fluorescent coloring matter.
A predetermined voltage is applied to the thus constituted EL layer from a pair of electrodes, whereby the carriers are recombined together in the light-emitting layer to emit light. In this specification, the EL element is called to have been driven when it emits light.
In this specification, the light-emitting element formed by an anode, an EL layer and a cathode is called EL element.
FIG. 14 illustrates the structure of a representative active matrix-type EL display device (hereinafter referred to as EL display device), wherein FIG. 14(A) shows the arrangement of a pixel unit of the EL display device and a drive circuit therefor. Reference numeral 901 denotes a pixel unit, 902 denotes a source signal line drive circuit, 903 denotes a gate signal line drive circuit, and 905 denotes draw-out terminals.
The pixel unit 901 includes plural pixels 906. Reference numeral 904 denotes power source feed lines formed on the pixel unit 901 to apply a potential to the pixel electrodes of the EL elements possessed by all pixels 906. Power source feed lines 904 are connected to detour wirings 907 which are connected to an external power source via draw-out terminals 905.
Pixels 906 are selected by select signals input to gate signal lines 913 from the gate signal line drive circuit 903. The potential of the power source feed lines 904 is given to the selected pixels 906 due to video signals input to the source signal line 912 from the source signal line drive circuit 902, and the pixels 906 display part of the picture.
FIG. 14(B) is a circuit diagram of pixels corresponding to R (red), G (green) and B (blue) among the pixels 906 shown in FIG. 14(A).
In FIG. 14(B), a pixel 906r for R, a pixel 906g for G and a pixel 906b for B have a common gate signal line 913. Further, the pixel 906r for R has a source signal 912r for R, the pixel 906g for G has a source signal line 912g for G, and the pixel 906b for B has a source signal line 912b for B.
The pixel 906r for R, the pixel 906g for G and the pixel 906b for B have a switching TFT 910 and an EL drive TFT 911, respectively. Further, the pixel 906r for R has an EL element 915r for R, the pixel 906g for G has an EL element 915g for G, and the pixel 906b for B has an EL element 915b for B.
When a select signal is input to the gate signal line 913, the switching TFTs 910 connected at their gate electrodes to the gate signal line 913 are all turned on. In this specification, this state is referred to as that the gate signal line 913 is selected.
Video signals input to the source signal line 912r for R, to the source signal line 912g for G and to the source signal line 912b for B, are further input to the EL element 915r for R, to the EL element 915g for G and to the EL element 915b for B through the switching TFTs 910 which have been turned on, so as to be input to the gate electrodes of the EL drive TFTs 911.
When the video signals are input to the gate electrodes of the EL drive TFTs 911, the potential of the power source feed line 914r for R is applied to the pixel electrode of the EL element 915r for R, the potential of the power source feed line 914g for G is applied to the pixel electrode of the EL element 915g for G, and the potential of the power source feed line 914b for B is applied to the pixel electrode of the EL element 915b for B. As a result, the EL element 915r for R, the EL element 915g for G and the EL element 915b for B emit light, and a display is produced by the pixel 906r for R, by the pixel 906g for G and by the pixel 906b for B.
The EL display devices can be roughly divided into those of the four color display systems, such as those of the system shown in FIG. 14 forming EL elements of three kinds of organic EL materials corresponding to R (red), G (green) and B (blue), those of the system in which white light-emitting EL elements and color filters are combined together, those of the system in which blue or bluish green light-emitting EL elements and a fluorescent material (fluorescent color-conversion layer: CCM) are combined together, and those of the system in which EL elements corresponding to RGB are stacked by using transparent electrodes for the cathodes (opposing electrodes).
In general, even when the same voltage is applied to the EL layer, the light-emitting brightness of the EL layer differs depending upon the organic EL material used for the EL layer. FIG. 15 illustrates voltage—brightness characteristics of the EL layers of each of the colors. As shown in FIG. 15, the light-emitting brightness of the EL layer to the applied voltage, varies depending upon the organic EL materials used for the EL elements of each of the colors. This is because the current density of when the same voltage is applied differs depending upon the organic EL materials.
Even when the current density remains the same, the light-emitting brightness differs depending upon the organic EL materials.
In the EL display device, therefore, the potentials of the power source feed lines for the pixels of each of the colors are usually adjusted to maintain a balance in the light-emitting brightness of the EL elements of three colors.
The magnitude of electric current flowing into the pixel unit through the detour wiring is determined by the number of pixels producing a white display in the pixel unit. The pixels producing the white display stand for those pixel elements having an EL element which is in a light-emitting state. The electric current that flows into the pixel unit via the detour wiring increases with an increase in the number of the pixels producing the white display.
An increase in the current flowing through the detour wiring results in a drop in the potential through the detour wiring. Therefore, the voltage applied to each EL element becomes small and the light-emitting brightness of each pixel decreases when an increased number of pixels are producing the white display.
In the case of the color EL display device, in particular, the voltages applied to the EL elements of each of the colors are adjusted to change the magnitudes of current flowing into the EL elements of each of the colors. An increase in the currents flowing into the pixels results in an increase in the drop of potential through the detour wirings to the corresponding pixels. Therefore, even when the voltages applied to the EL elements of each of the colors are adjusted, the ratio of currents flowing into the EL elements of three colors changes depending upon whether the number of the pixels producing the white display is large or not.
A change in the number of the pixels producing the white display invites a loss of balance in the light-emitting brightness of the pixels corresponding to the three colors.
In the conventional EL display device, the magnitude of current that flows into the EL elements changes depending upon the colors, and different voltages are applied to the EL elements. However, the EL drive TFTs provided as switching elements between the EL elements and the power source feed lines have the same LDD width and the same channel width and, besides, voltages of the digital signals input to the gate electrodes of all EL drive TFTs have the same amplitude. Accordingly, the EL drive TFTs are deteriorated by the magnitudes of voltages applied to the power source feed lines. Besides, when the amplitudes of voltages of digital signals input to the gate electrodes of the EL drive TFT gate electrodes are too great, it becomes difficult to suppress the consumption of electric power.